Animals, such as mammals and birds, are often susceptible to parasite infestations. These parasites may be ectoparasites, such as fleas and ticks. Animals and humans also suffer from endoparasitic infections including, for example, helminthiasis which is most frequently caused by a group of parasitic worms described as nematodes or roundworms. These parasites cause severe economic losses in pigs, sheep, horses, and cattle as well as affecting companion animals (e.g. cats and dogs) and poultry. Other parasites include those which occur in the gastrointestinal tract of animals and humans include Ancylostoma, Necator, Ascaris, Strongyloides, Trichinella, Capillaria, Toxocara, Toxascaris, Trichuris, Enterobius and parasites which are found in the blood or other tissues and organs such as filarial worms and the extra intestinal stages of Strongyloides, Toxocara and Trichinella. 
One type of endoparasite which seriously harms mammals is Dirofilaria immitis, also known as Heartworm. Other filarial endoparasites include Dirofilaria repens and Dirofilaria honkongensis, which can also infect humans. The most common hosts are dogs and cats but other mammals such as ferrets and raccoons may also be infected. Heartworms go through several life stages before they become adults infecting the pulmonary artery of the host mammal. The worms require the mosquito as an intermediate host to complete their life cycle. The period between the initial infection when the dog is bitten by a mosquito and the maturation of the worms into adults living in the heart and pulmonary arteries is six to seven months in dogs and is known as the “prepatent period”. L3 larvae migrate during blood feeding of the mosquito to the tip of the mosquito's mouth parts (labium), leave the mosquito and are deposited on the skin of the dog where they then migrate through the bite wound into the host. Most L3 larvae molt to fourth-stage larvae (L4s) in canine subcutaneous tissues within 1-3 days after infection. Then, they migrate to the muscles of the chest and abdomen, and 45 to 60 days after infection, molt to the fifth stage (L5, immature adult). Between 75 and 120 days after infection, these immature heartworms then enter the bloodstream and are carried through the heart to reside in the pulmonary artery. Around seven months after infection, Dirofilaria immitis adults reach maturity and sexually reproduce in the pulmonary arteries and right ventricle. Adult males are around 15 cm in length, and females are around 25 cm in length and their normal life span as adults is calculated to be about 5 years.
Heartworm infection is a severe and life-threatening disease. Canine heartworm infection is preventable and prophylaxis treatment is a priority in heartworm endemic areas. Treatment of mature heartworm infection with an adulticide (e.g. melarsomine dihydrochloride) is costly and can cause serious adverse side effects, thus prevention by monthly administration of drugs that interrupt larvae development is widely used. The goal of marketed heartworm preventive therapies in dogs is to prevent the development of the parasite to adult heartworms by interrupting the Dirofilaria immitis life cycle post-infection.
The macrocyclic lactones (MLs, e.g. ivermectin, eprinomectin, milbemycin oxime, moxidectin, and selamectin) are the most commonly used chemoprophylaxis agents and are administered at monthly or six-month intervals. These drugs have been effective against Dirofilaria immitis infective third-stage larvae (L3) deposited by the mosquito as well as maturing fourth-stage larvae (L4). When administered monthly, MLs kill L3 and L4 larvae acquired within the previous 30 days, and thus prevent disease caused by adult worms. MLs can also be used monthly in infected dogs to suppress reproduction in adult worms and remove microfilariae, thereby reducing transmission and gradually causing the attrition of adult worms (Vet. Parasitol. 2005 Oct. 24 133(2-3) 197-206).
In recent years, an increased number of lack of efficacy (LOE) cases have been reported, in which dogs develop mature heartworm infections despite receiving monthly prophylactic doses of macrocyclic lactones drugs. For example, Atkins et al., (Vet. Parasitol. 206 (2014) 106-113) recently reported that an increasing number of cases of dogs that tested heartworm antigen positive while receiving heartworm preventive medication which suggests that some populations of Dirofilaria immitis have developed selectional resistance to heartworm preventives (American Heartworm Society, 2010. Heartworm Preventive Resistance. Is it Possible, vol. 37. Bulletin of the American Heartworm Society, pp. 5.). Thus, there is an ongoing need to develop new anthelmintic agents with improved activity against Dirofilaria immitis and other endoparasites.
Various parasiticides exist in the art for treating endoparasites infections in animals. In addition to the macrocyclic lactones, cyclic depsipeptides with antiparasitic activity are known. PF1022A, a 24-membered cyclooctadepsipeptide isolated from the fungus Mycelia sterilia by Sasaki et al. (see J. Antibiotics 45: 692-697 (1992)), has been found to exhibit broad anthelmintic activity against a variety of endoparasites in vivo with low toxicity. These compounds are described, for example, in U.S. Pat. Nos. 5,514,773; 5,747,448; 5,646,244; 5,874,530; among others, which are incorporated herein by reference. Emodepside is a semi synthetic analog of PF1022A containing a morpholine group at the para position of the aryl ring in the phenyl lactate groups. Emodepside is a potent anthelmintic used in combination with praziquantel in the product Profender® for the treatment of parasitic worms in cats and dogs. However, the antiparasitic activity of PF1022A and emodepside is not satisfactory for the treatment of certain parasites, especially for the control of Dirofilaria immitis in mammals to prevent the establishment of heartworm disease. Thus, there is a need in the art for more effective antiparasitic agents for treatment and protection of animals, e.g. mammals, fish and birds against parasites, in particular internal parasites including nematodes and filarial worms such as heartworm.
It is expressly noted that citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention. Any foregoing applications, and all documents cited therein or during their prosecution (“application cited documents”) and all documents cited or referenced in the application cited documents, and all documents cited or referenced herein (“herein cited documents”), and all documents cited or referenced in herein cited documents, together with any manufacturer's instructions, descriptions, product specifications, and product sheets for any products mentioned herein or in any document incorporated by reference herein, are hereby incorporated herein by reference, and may be employed in the practice of the invention.